With the highly development of the internet, various kinds of data transmission on the internet are required. Therefore, a diversity of communication and transmission means are developed and used to facilitate the work on the internet. Particularly for the means requiring large bandwidth for supporting serial data flow, e.g. digital display, high resolution frame or color graphics, the transmission of the large amount of data relies much on the analog technique to design circuit system and support data transmission. Low voltage differential signaling (LVDS) is one of the analog techniques for transmission. This technique can be utilized to design hybrid signaling systems. The LVDS technique is a high-speed analog circuiting technique, which supports data transmission on copper wires in a multi-gigabit order.
Since LVDS transmission technique is a universal interface standard for high-speed, low-power and low noise data transmission, it can be well applied to the system structure requiring modulating digital image signals into analog image signals and transmitting the analog image signals to a digital display at a high rate.
Please refer to FIG. 1, which is a functional block diagram schematically showing a digital display system using LVDS transmission technique. The system comprises a computer host 10 and a display host 20 such as a liquid crystal display (LCD). An image control chip 101 inside the computer host 10 outputs a digital image signal S1 to an LVDS transmitter 102 to be modulated into an analog image signal S10 having a swing as low as 300 mV˜350 mV. Then, via an LVDS driving circuit 1021, the analog image signal S10 is outputted to an LVDS receiver 201.
In order to modulate and recover the analog image signal S10 into the digital image signal S1 capable of being processed by the downstream devices such as a timing control device 202 and a display driving device 203, at least two resistors (FIG. 2, R1 and R2) are arranged at the input of the LVDS receiver 201.
The circuit of a conventional LVDS driving circuit 1021 is shown in FIG. 2. The skeleton of the circuit is a differential amplifier comprising of four transistors Q1˜Q4, from which the analog image signal S10 is generated and outputted to the resistors R1 and R2 of the LVDS receiver 201. On the other hand, the transistor Q5 and the parallelly interconnected transistors Q6 and Q7 constitute a first and a second current sources, respectively, wherein the first current source is electrically connected to a power source +Vcc and the differential amplifier, and the second current source is electrically connected to the differential amplifier and ground. By way of the first and the second current sources, a constant source current Io is provided for the differential amplifier. The voltage state of the constant current source can be stabilized with the presence of the transistor Q8.
As is understood by those skilled in the art, the properties of the transistors Q5, Q6 and Q7, which serve as the first and the second current sources, likely vary with some certain or uncertain factors during the manufacturing process. Once this happens, the constant source current Io provided for the differential amplifier comprising of the transistors Q1˜Q4 will deviate from the originally designed specification so as to make the voltage swing Va of the analog image signal S10 outputted by the differential amplifier beyond the predetermined range. Then, the erroneous operation of the downstream LVDS receiver 201 may be rendered. Unfortunately, the signal drift problem resulting from the manufacturing factors of the above-mentioned LVDS driving circuit 1021 cannot be solved in the conventional LVDS transmitter 102.